Roller hemming is a relatively recent development for joining inner and outer body panels by folding the outer flange over the edge of the inner panel. This process can create a sharp hem appearance comparable to conventional hemmers for typical steel sheet panels. However, when hemming aluminum panels, conventional hemmers must be modified to reduce the bending severity of the aluminum sheet in order to prevent cracking along the hemline. Sharp, flat hems are very difficult to produce in aluminum panels with conventional hemmers.
A roller hemming apparatus deforms the flange through a non-plane strain deformation path which enables more severe bending in aluminum sheet to possibly form a flat hem, similar to steel panels. In either material, the cycle time for roller hemming is generally much longer than with conventional hemming because the roller must bend the flange in two to three passes around the periphery of the panel. The initial pass bends the flange to the prehem position (approximately 45 degrees), while the final pass flattens the hem.
Even with this advantage, age-hardenable aluminum alloys, such as AA6111 commonly used for outer panels, have very limited bendability, especially when material at the hemline is pre-strained during the stamping operation. The retrogression heat treatment (RHT) process of U.S. Pat. No. 5,948,185 has been used in production of aluminum panel assemblies to improve hemming and prevent cracking with conventional hemmers.
The RHT process applies a local heat treatment and immediate quench to the flange area that temporarily softens the material by dissolving unstable particles in the microstructure. This provides sufficient bendability to flat hem age-hardenable aluminum panels. A disadvantage of RHT is inserting the heat treatment operation in the production flow between the flanging press and the marriage station. The heat treatment is performed in a separate operation, which adds a process step that increases the size of the assembly cell and increases manufacturing cost.
Current roller hemming methods use a solid, free rolling tool on the end of a robotic mount section to deform a sheet metal flange to a flat hem condition by rolling around the perimeter of a panel in a series of passes. These methods bend the flange from 90-degrees open to completely closed and flat by deforming the sheet metal through a non-plane strain bending path.
Conventional hemmers bend the flange through “plane strain bending,” which is the most severe strain path and often leads to cracking at the hemline of aluminum panels. With the alternate strain path bending, roller hemming is capable of flat hemming aluminum panels without splitting. However, tight radius bending with the roller to achieve a desirable sharp, crisp, appearance may not be possible for aluminum alloys without specially treating the material, as in the case of RHT (retrogression heat treatment), which locally and temporarily softens the material in the bend region.